In jack, I had alsa-capture connected to tremolo_in and tremolo_out connected to alsa-playback. I balanced the pcm and line-in sliders in mixer to get some of the effect into the sound, without the feedback. While running, the bpm seemed to vary erratically. For 10 minutes or so, however, the tremolo worked. Then it stopped working. There was no special error messages from the ecasound engine. Feedback was still there when I put pcm volume too high, but no tremolo. I tried it couple of times, but it was always the same.

Then I tried:

# ecasound -f 16,1,44100 -i alsa -eemb 400,400 -ea 400 -o alsa -b 64

which worked, although it was much harsher sounding than the jacked version.